The capture of solar energy, in terms of the thermal capture aspect, is of increasing technological and economic importance from the point of view of hot water production, heating and refrigeration at a domestic level as well as electricity generation in solar thermoelectric power plants.
These systems require maximum solar energy absorption and the minimum possible loss of energy. With this end in mind, these systems are configured in vacuum tubes or similar structures that reduce losses by conduction and convection and possess coatings with a great capacity for solar energy absorption and low emissivity characteristics to reduce energy losses through thermal radiation in far infrared.
Consequently, in the domestic area as well as in the production of electricity, selective absorbent coatings play an essential role. There are numerous records of absorbent coatings such as those described in patents WO2005/121389, U.S. Pat. Nos. 4,582,764, 4,628,905, 5,523,132, US2004/0126594, US2005/0189525, US2007/0209658, WO97/00335, as well as several others. In all of these the absorbent coating consists of a metallic layer that provides low emissivity characteristics, one or more layers of dielectric materials doped with “Cermets” metallic elements, which act as absorbent layers for solar radiation and a dielectric layer that provides anti-reflective properties. In some of these there is also an additional dielectric layer that acts as a blocking layer against the diffusion of different materials. The layers of cermets are absorbent layers, complex refraction index, where the capacity for absorption is provided by the co-dopant metal element whose concentration can be constant or gradual within each of the layers.
Cermets are habitually oxides or metal nitrides doped with metallic elements such as Mo, Ni, Ti, Ta, Al, etc., that are usually deposited through codeposition techniques through reactive cathodic spraying “reactive sputtering”. Codeposition through reactive sputtering consists in simultaneous evaporation of two materials through sputtering in the presence of, in addition to the inert gas, a reactive Gas, oxygen, nitrogen etc., residual in the deposition chamber. The residual gas reacts with one of the evaporated materials forming the corresponding dielectric compound, while part of the other compound is deposited in metal form. Reactive gas reacts both with the material that forms the dielectric compound and the doping metal, so to obtain the cermets with the appropriate absorption a stringent control of the stoichiometry process is required. The stoichiometry process is conditioned by the composition and partial pressures of the gases in the vacuum chamber in relation to the reactive gas consumption, and therefore depends on the evaporation speed, that is, cathode power, of its state, making accurate control of the stoichiometry process very difficult, requiring certain feedback mechanisms that can negatively affect the properties of the coatings.
Likewise, part of the codopant metal also reacts with the reactive gas and forms dielectric compounds, so this metal does not therefore contribute to the absorption of the layer and requires large concentrations of codopant metal. Besides, this technique also presents limitations when choosing codopant metals as they should possess much less affinity for the reactive gas than the principal metal that forms the dielectric compound.